Capacitance-type liquid sensor

ABSTRACT

A capacitance type liquid sensor is disclosed which detects a tilt angle and acceleration of an object using the fact that a liquid surface always keeps itself horizontal. Openings ( 13, 14 ) are formed in two sides of a hollow cylindrical closed container ( 6 ) made of an electrically insulating material, and the container has two parallel sides ( 2, 3 ). Plate-shaped main electrodes ( 4, 5 ) on at least one face of each of which silicon oxide film is formed are made to be in contact with the sides so as to close the openings, with the silicon oxide film being placed so as to face the inside of the container. A sealing agent ( 28 ) is interposed in a gap between the plate-shaped main electrodes and the sides. The container is filled with electrically conductive liquid ( 27 ) of an amount equal to substantially one-half of the inside volume of the container. An auxiliary electrode ( 8 ) brought into electrical contact with the conductive liquid is mounted in the container.

TECHNICAL FIELD

The present invention relates to a capacitance type liquid sensor whichuses an electrically conductive liquid and is suitable for use as tiltsensors, acceleration sensors and seismoscopes.

BACKGROUND ART

A tilt sensor is used for measurement of a tilt angle of a measuredobject on which the tilt sensor is mounted, relative to a horizontalsurface or a vertical axis of the object. On the other hand, anacceleration sensor and a seismoscope are used for measurement ofacceleration received by an object on which the sensor is mounted. Sinceboth types of sensors differ in an object to be measured, differentsensors suitable for measurement of respective physical quantities areusually used selectively.

However, a liquid sensor using an electrically conductive liquid hasconventionally been known as usable for measurement of both tilt angleand acceleration. The liquid sensor utilizes a principle that a liquidsurface always keeps itself horizontal in a stationary state. When acontainer of the sensor containing the liquid therein tilts, an angle ofthe container relative to the liquid surface in the container isdetected so that a tilt angle of the container is measured. Conversely,when a horizontal acceleration is applied to the sensor containerdisposed horizontally, the liquid surface is tilted in the container, sothat a tilt angle of the liquid surface is detected, whereby appliedacceleration is measured. In this specification, a sensor measuring atilt angle or an acceleration using such a principle is referred to as“liquid sensor.”

A resistance type and a capacitance type are known as a method or systemof detecting a tilt angle between a surface of liquid contained in acontainer and the container.

JP-A-2001-13160 discloses a technique pertaining to the resistance type,for example. In this sensor, as shown in the longitudinal section ofFIG. 20, a suitable amount of electrically conductive liquid 102 isenclosed in a circular cylindrical metal container 101 having a closedend, and an opening is closed by a metal disc 103. One or two pairs ofmetal electrodes 104 extend through and are fixed in the disc 103 in anelectrically insulated manner. When the container 101 tilts or ahorizontal acceleration is applied to the container 101, an anglebetween the container 101 and a surface 105 of the liquid in thecontainer is changed such that a contact area of the metal electrode 104with the conductive liquid 102 is changed, whereupon an electricalresistance is changed between the metal container 101 and each metalelectrode 104. Accordingly, changes in the resistance value are measuredso that a tilt angle of the container 101 or a magnitude of accelerationis detected.

In the case of the resistance type liquid sensor, however, the metalelectrodes are in direct contact with the conductive liquid(electrolytic solution) and accordingly, electric current for themeasurement of resistance flows through a boundary therebetween. Thisresults in a chemical change such as elution of metal composing theelectrode or electrolysis of conductive liquid. As a result, therearises a problem that it is difficult to secure stability andreliability of the sensor for a long period of time.

On the other hand, JP-A-5-172571 discloses a technique pertaining to thecapacitance type liquid sensor, for example. As shown in FIG. 21, acontainer 112 includes a cylindrical frame 110 disposed horizontally andmade from a conductive material. The frame 110 has both open ends closedby insulating plates 111. The container 112 is filled with a conductiveliquid 113 whose amount corresponds to substantially one half ofcapacity of the container and an insulating liquid 114 having a smallerspecific gravity than the conductive liquid 113. Arc outer electrodes115 formed by dividing outer surfaces of a semicircular or circularshape are provided on outer surfaces of the insulating plates 111respectively. When the container 113 tilts, a capacitor constituted bythe both end insulating plates 111 sandwiched between an arc outerelectrode 115 and conductive liquid 113 changes a capacitance thereof.An amount of change is measured so that a tilt of the container isdetected.

In the capacitance type sensor, however, a usual insulating plate isused as a dielectric forming the capacitor. Since the insulating plateconstitutes a part of the container 112, its thickness cannot be reducedso much. Accordingly, since it is difficult to render the capacitance ofthe capacitor larger and the detectivity is low, there arises a problemthat it is difficult to increase the detection accuracy.

Further, JP-A-11-118412 discloses a displacement signal generatingdevice of the capacitance type. The device includes a single electrodesubstance and two electrodes of respective dielectric structural bodieshaving the electrodes such as a chemically treated aluminum foil, allthe electrodes being impregnated in an electrolyte contained in acontainer. Electric elements are electrically connected between thesingle electrode substance and the electrodes of the dielectricstructural bodies. When the container filled with the electrolyte oreach of the electrodes of the dielectric structural bodies is displaced,a contact area of each of the electrodes of the dielectric structuralbodies with the electrolyte is changed such that a capacitance betweenthe single electrode substance and each of the electrodes of thedielectric structural bodies is changed. The change in the capacitanceis measured, whereby displacement is detected.

In the capacitance type sensor, however, the chemically treated aluminumfoil is used as the electrodes of the dielectric structural bodies. Asurface of aluminum foil is anodized so that an aluminum oxide coatserving as a dielectric is formed thereon. Since the chemically treatedaluminum coat has a problem in the stability, it is difficult to securethe stability and reliability of the sensor for a long period of time.

DISCLOSURE OF THE INVENTION

The present invention was made to overcome the above-described problemsin the prior art and an object thereof is to provide a capacitance typeliquid sensor which can maintain the stability and reliability for along period of time.

The object of the present invention is accomplished by providing acapacitance type liquid sensor comprising a cylindrical closed containermade from an electrically insulating material and having two sidesparallel to each other, the sides having respective openings formedtherein, the container being filled with a conductive liquid, aplurality of plate-shaped main electrodes each having at least one sideformed with a silicon oxide film, the main electrodes being in abutmentwith said sides while the silicon oxide films face an inside of thecontainer, thereby closing the openings, a sealing agent interposed in agap between the main electrodes and said sides for fluid tightnessrespectively, and an auxiliary electrode provided in the container to bebrought into electrical contact with the conductive liquid, wherein theconductive liquid has an amount corresponding to substantially one halfof a content volume of the container.

The liquid sensor having the above-described construction has anadvantage of being able to maintain stability and reliability for a longperiod of time and to be manufactured into a miniature size since a verythin silicon oxide film superior in the electric and chemical stabilityis used as a dielectric forming a capacitor.

Further, the object of the present invention is also accomplished byproviding a capacitance type liquid sensor comprising a closedcontainer, an electrically conductive liquid having an amount equal tosubstantially one-half of an inside volume of the container and fillingthe container, a pair of lead terminals extending through one of ends ofthe container and fixed to the container so as to be electricallyinsulated from the container, a plurality of main electrodes havingsurfaces formed with silicon oxide films and mounted on distal ends ofthe lead terminals, respectively, the main electrodes being provided sothat parts of the main electrodes are located on a liquid surface of theconductive liquid when the container is stationary, and an auxiliaryelectrode electrically conductively brought into contact with theconductive liquid.

The liquid sensor having the above-described construction also has theadvantage of being able to maintain stability and reliability for a longperiod of time and to be manufactured into a miniature size since a verythin silicon oxide film superior in the electric and chemical stabilityis used as a dielectric forming a capacitor.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a longitudinal section of the liquid sensor in accordance witha first embodiment of the present invention;

FIG. 2 is a perspective view of the liquid sensor in accordance with thefirst embodiment of the present invention;

FIG. 3 is a front view of the liquid sensor in accordance with the firstembodiment of the present invention;

FIG. 4 is a transverse section of the liquid sensor in accordance withthe first embodiment of the present invention;

FIG. 5 is an explanatory view in the case where the liquid sensor inaccordance with the first embodiment of the present invention is used asa tilt sensor;

FIG. 6 is an explanatory view in the case where the liquid sensor inaccordance with the first embodiment of the present invention is used asan acceleration sensor;

FIG. 7 is an example of an AC bridge circuit converting into voltage thedifference of capacitance of a capacitor on the main electrode surface;

FIG. 8 is a perspective view of the liquid sensor in accordance with asecond embodiment of the present invention;

FIG. 9 is a transverse section of the liquid sensor in accordance withthe second embodiment of the present invention;

FIG. 10 is a transverse section showing another embodiment of a mannerof mounting a main electrode to a cylindrical container;

FIG. 11 is a longitudinal section of the liquid sensor in accordancewith a third embodiment of the present invention;

FIG. 12 is a perspective view showing arrangement of electrodes in thecontainer of the liquid sensor in accordance with the third embodimentof the present invention;

FIG. 13 is an explanatory view in the case where the liquid sensor inaccordance with the third embodiment of the present invention is used asa tilt sensor;

FIG. 14 is an explanatory view in the case where the liquid sensor inaccordance with the third embodiment of the present invention is used asan acceleration sensor;

FIG. 15 is a perspective view showing arrangement of electrodes in thecontainer of the liquid sensor in accordance with a fourth embodiment ofthe present invention;

FIG. 16 is a perspective view showing another embodiment of theelectrode arrangement in the liquid sensor in accordance with the thirdembodiment of the present invention;

FIG. 17 is an explanatory view in the case where the liquid sensorhaving the electrode arrangement as shown in FIG. 16 is used as anacceleration sensor;

FIG. 18 is a perspective view showing another embodiment of theelectrode arrangement in the liquid sensor in accordance with the fourthembodiment of the present invention;

FIG. 19 is a longitudinal section in the case where the liquid sensor inaccordance with the third and fourth embodiments is provided with anauxiliary electrode extending through the lower disc;

FIG. 20 is a longitudinal section of an example of resistance typeliquid sensor in accordance with the prior art; and

FIG. 21 is an example of capacitance type liquid sensor in accordancewith the prior art.

BEST MODE FOR CARRYING OUT THE INVENTION

The present invention will be described with reference to theaccompanying drawings in order that the invention may be rendered clearin more detail.

First Embodiment

A first embodiment of the present invention will be described withreference to the accompanying drawings. FIG. 2 is a perspective view ofthe liquid sensor of the embodiment, FIG. 3 is a front view thereof,FIG. 1 is a longitudinal section thereof (sectional view taken alongline A-A), and FIG. 4 is a transverse section thereof (sectional viewtaken along line B-B).

The liquid sensor 1 of the embodiment has such a structure that openings13 and 14 are provided in two parallel sides 2 and 3 of a cylindricalcontainer 6 with both closed ends, plate-shaped main electrodes 4 and 5having surfaces covered with silicon oxide films are fitted in theopenings 13 and 14 thereby to close them, an interior of the containeris filled with a suitable amount of conductive liquid 7, and a metalauxiliary electrode bar 8 is inserted through an upper lid 11 into theconductive liquid 7.

The container 7 is a cylindrical one and has two parallel sides 2 and 3.The sides are required to include two parallel faces and a transversesection need not be rectangular. Upper and lower openings 9 and 10 areclosed by upper and lower lids 11 and 12 respectively. The cylindricalcontainer 6 is made from an electrically insulating material such asceramic, hard glass, synthetic resin, etc. with the upper and lower lids11 and 12.

The parallel sides 2 and 3 are formed with vertically extending slenderrectangular openings 13 and 14 respectively. Four end faces 15 and 16defining the openings 13 and 14 have all the outer corners 17 and 18 cutout in the direction of extension of the end faces 15 and 16 so that thecorners have rectangular sections, whereby notches 19 and 20 are formedso that plate-shaped members are fitted into the notches. As the resultof formation of the notches 19 and 20, sectional areas of the openings13 and 14 at the inside of the container are smaller than sectionalareas of openings 21 and 22 at the outside of the container with aboundary at the middle in the direction of thickness of the sides 2 and3.

The plate-shaped main electrodes 4 and 5 are fitted in the openings 21and 22 respectively. The plate-shaped main electrodes 4 and 5 are eachmade of silicon (Si) and each have at least one side formed with asilicon oxide film on its entirety. The plate-shaped main electrodes 4and 5 have areas larger than sectional areas of the openings 13 and 14at the inside of the container and slightly smaller than sectional areasof the openings 21 and 22 at the outside of the container, respectively.The plate-shaped main electrodes 4 and 5 are fitted in the openings 21and 22 so that the sides with the silicon oxide films abut against thebottoms 23 and 24 of the notches 19 and 20, respectively. Lead wires 25and 26 for taking out the potential of the main electrodes are connectedby a conductive paste, soldering or the like to electrode surfaceportions from which the silicon oxide film has been removed.

The silicon (Si) which is the material for the main electrodes 4 and 5includes single crystal silicon, amorphous silicon, polycrystallinesilicon, etc. Further, the silicon oxide film can be formed by anordinary method used in the IC production process, such as the thermaloxidation process, CVD process or the like.

When the plate-shaped main electrodes 4 and 5 are fitted in therespective openings 21 and 22, gaps in abutting portions of the mainelectrodes 4 and 5 and the notches 19 and 20 are filled with a sealingagent 28 such as low-melting glass, synthetic resin bonding agent or thelike. Further, gaps between the outer peripheral faces of the mainelectrodes 4 and 5 and the sides of the notches 19 and 20 are alsofilled with the sealing agent 28. Thus, the main electrodes 4 and 5 arefixed to the cylindrical container 6 for fluid tightness.

The closed cylindrical container 6 is filled with an electricallyconductive liquid 7 whose amount is equal to substantially one-half ofan inside volume of the cylindrical container 6. The metal auxiliaryelectrode bar 8 extends through the upper lid 11 and is fixed so that adistal end thereof is soaked sufficiently deep in the conductive liquid7.

When brought into contact with the conductive liquid 7, the sealingagent 28 is swollen such that there is a possibility that the adhesionmay be reduced or the composition of the sealing agent 28 may beresolved into the conductive liquid 7 such that the electricalconductivity would change. Accordingly, it is desirable that a contactarea of the sealing agent 28 with the conductive liquid 7 should be assmall as possible. For this purpose, when the main electrodes 4 and 5are fitted in the respective openings 13 and 14, a suitable amount ofsealing agent 28 is applied to the bottoms 23 and 24 of the notches soas not to be forced out, and the main electrodes are fitted in therespective openings while pressed against the respective openings.Subsequently, in order that the main electrodes 4 and 5 may securely befixed, a sufficient amount of sealing agent 28 is applied to outerperipheral faces of the main electrodes 4 and 5 and portions near theopenings adjacent to the outer peripheral faces so as to rise up.

Consequently, a portion where the sealing agent 28 is brought intocontact with the conductive liquid 7 containing alcohol as a maincomposition is limited to one interposed in small gaps between the mainelectrodes 4 and 5 and the closed container 6. Accordingly, the adhesioncan be prevented from being reduced due to the swell of the sealingagent 28 or the electrical conductivity can be prevented from beingreduced due to solution of the composition of the sealing agent 28 intothe conductive liquid 7.

Since the electrical conductivity of the conductive liquid 7 needs to berendered sufficiently high, an electrolyte such as lithium nitrate,potassium iodide or the like is dissolved into a solvent. A suitablesolvent includes an alcohol group including methyl alcohol, ethylalcohol and isopropyl alcohol, a ketone group including acetone andmethyl ethyl ketone, an ether group including diethylene glycolmono-butyl ether and the like. These solvents may be used individuallyor a plurality of the solvents may be combined together in use.

What solvent and electrolyte are suitable depends upon the usage of theliquid sensor. For example, when the liquid sensor is used as aseismoscope measuring a magnitude of earthquake, acceleration changesrepeatedly in the positive and negative directions at a frequency ofseveral Hz. In order that a periodically changing acceleration may bemeasured, the responsibility to be able to sufficiently follow thefrequency is required. In addition, the sensor needs to be designed sothat a resonant frequency thereof does not correspond with a frequencyof earthquake. In order that these conditions may be satisfied, theconductive liquid 7 necessitates the conditions of specific gravity,viscosity, surface expansion coefficient and the like. These conditionsdepend mainly upon the relationship with an inner sectional form of thecontainer 6. Accordingly, types of a solvent and electrolyte to be usedare determined in view of the aforesaid requirements, a servicetemperature range and the like.

Additionally, an upper remaining space inside the container 6 is filledwith an inert gas.

The operation of the liquid sensor 1 thus constructed will now bedescribed. The surfaces of the main electrodes 4 and 5 facing the insideof the container are covered with the respective silicon oxide films.Since the silicon oxide films are dielectric, a parallel plate capacitorC1 and a parallel plate capacitor C2 are formed. In the capacitor C1,the main electrode 4 serves as one electrode and the conductive liquid 7serves as the other electrode with the silicon oxide film beinginterposed therebetween. In the capacitor C2, the main electrode 5serves as one electrode and the conductive liquid 7 serves as the otherelectrode with the silicon oxide film being interposed therebetween.

A capacitance C1 of the capacitor C1 and a capacitance C2 of thecapacitor C2 are shown by the following equations:C1=∈·S1/t andC2=∈·S2/twhere S1 and S2 are contact areas of the main electrodes 4 and 5 and theconductive liquid 7 respectively, t is a thickness of each silicon oxidefilm and ∈ is a dielectric constant.

More specifically, the values of capacitances C1 and C2 of thecapacitors C1 and C2 are calculated from the contact areas S1 and S2 ofthe main electrodes 4 and 5 and the conductive liquid 7. Conversely, thecontact areas S1 and S2 of the main electrodes 4 and 5 and theconductive liquid 7 can be obtained by calculation when the values ofthe capacitances C1 and C2 are known.

Using the above relationship, the case where the liquid sensor 1 of theembodiment is used as a tilt sensor will be described. Consider now thecase where the container 6 is placed so that a central axis thereof isperpendicular to the liquid surface 27 of the conductive liquid 7 asshown in FIG. 1. In this state, the contact areas S1 and S2 of the mainelectrodes 4 and 5 and conductive liquid 7 are equal to each other andaccordingly, the values of the capacitances C1 and C2 become equal toeach other.

Suppose now that the central axis of the cylindrical container 6 thentilts by a tilt angle θ from a vertical axis along the lineperpendicular to the surfaces of the main electrodes 4 and 5 as shown inFIG. 5. The contact area S1 of the main electrode 4 and conductiveliquid 7 is increased and accordingly, the value of the capacitance C1is increased. On the contrary, the contact area S2 of the main electrode5 and conductive liquid 7 is decreased and accordingly, the value of thecapacitance C2 is decreased. There is a constant relationship betweenthe difference between the contact areas S1 and S2 and the tilt angle θ.The relationship depends upon a configuration of the transverse sectionof the cylindrical container 6. Accordingly, when the difference betweenthe capacitances C1 and C2 is measured and the difference between thecontact areas S1 and S2, the value of tilt angle θ can be obtained bycalculation using the relational equations.

Next, a case where the liquid sensor 1 of the embodiment is used as anacceleration sensor will be described. The cylindrical container 6 isfixed to a horizontal substance so that the central axis becomesvertical in the same manner as shown in FIG. 1. Suppose now that ahorizontal acceleration is applied in the direction perpendicular to thesurfaces of the main electrodes 4 and 5 as shown in FIG. 6. If thecylindrical container 6 is not inclined, the conductive liquid 7 in thecontainer 6 is brought up to the side opposite to the direction ofacceleration by inertia, whereupon the liquid surface 27 tilts by thetilt angle .theta. from the horizontal position. As a result, thedifference arises between the capacitances C1 and C2 as in the use asthe tilt sensor. The difference is measured and accordingly, the tiltangle .theta. can be obtained. When the tilt angle .theta. is obtained,the magnitude of acceleration applied to the container 6 can be obtainedby calculation or using a previously obtained calibration curve. As theresult of the above-described operation, the liquid sensor 1 can be usedas the acceleration sensor. Since a seismoscope is one type of theacceleration sensor, the liquid sensor 1 can be used as the seismoscope.

The following describes a method of detecting the difference between thecapacitances C1 and C2 of the capcitors C1 and C2 formed on the surfacesof the main electrodes 4 and 5 respectively. FIG. 7 shows an AC bridgecircuit converting the difference between the capacitances C1 and C2 toa voltage change, thereby measuring the difference. Capacitors C3 and C4in the figure are fixed capacitors having respective capacitances equalto each other. A connecting point Y1 of the capacitors C1 and C2corresponds to the conductive liquid 7, and connecting points X1 and X2correspond to the lead wires 25 and 26 connected to outer surfaces ofthe main electrodes 4 and 5. The potential of the conductive liquid 7 istaken out by the auxiliary electrode bar 8.

A voltmeter 31 having a high internal impedance is connected between aconnecting point X1 of the serially connected capacitors C1 and C3 and aconnecting point X2 of the serially connected capacitors C2 and C4. AnAC voltage source 32 is connected between a connecting point Y2 of thecapacitors C3 and C4 and the connecting point Y1. The conductance of theconductive liquid 7 is adjusted so that a resistance value of theconductive liquid 7 is rendered sufficiently lower than impedances ofthe capacitors C1 and C2. Accordingly, the resistance value of theconductive liquid 7 can be ignored.

When an AC voltage is applied to the connecting points Yl and Y2 by theAC voltage source 32 and the difference between the capacitances of thecapacitors C1 and C2 is small, the voltmeter 31 indicates voltagesubstantially proportional to the difference of capacitances of thecapacitors C1 and C2. Accordingly, the difference of capacitances of thecapacitors C1 and C2 can be known by measuring the aforesaid voltage.When the difference of capacitances has been obtained, a tilt angle ofthe liquid sensor 1 or an acceleration applied to the liquid sensor 1can be obtained from the value of capacitance difference. Thecapacitance difference can be measured even when the capacitors C3 andC4 of the AC bridge circuit in FIG. 7 are replaced by fixed resistances.

Second Embodiment

The liquid sensor of the first embodiment can only measure a tilt angleor acceleration along the line perpendicular to the surfaces of thepaired main electrodes. However, a liquid sensor of a second embodimentcan measure a tilt angle or acceleration along two directionsintersecting each other in a horizontal face. FIG. 8 is a perspectiveview of the liquid sensor 1 a of the second embodiment and FIG. 9 is atransverse section of the liquid sensor. Identical or similar parts ofthe liquid sensor 1 a are labeled by the same reference symbols as thoseof the liquid sensor 1.

The liquid sensor 1 a of the second embodiment includes the cylindricalcontainer 6 formed into a square cylindrical shape. Four plate-shapedmain electrodes (4, 5, 4 a, 5 a) have four sides on which silicon oxidefilms are mounted, respectively, in a same manner as in the firstembodiment. The liquid sensor 1 a differs from the liquid sensor 1 ofthe first embodiment in that plate-shaped electrodes 4 a and 5 a areadditionally provided and is the same as the liquid sensor 1 of thefirst embodiment in the other respects. A mounting structure formounting the plate-shaped electrodes 4, 5, 4 a and 5 a is the same asthat in the first embodiment.

The plate-shaped electrodes 4, 5, 4 a, 5 a are disposed as describedabove such that a tilt angle or angular speed can be measured along aline perpendicular to the surfaces of the electrodes 4 and 5simultaneously with the tilt angle or angular speed along the lineperpendicular to the surfaces of the main electrodes 4 and 5. Morespecifically, a tilt angle and an acceleration along two directionsintersecting each other in the horizontal face can be measuredsimultaneously. Accordingly, the tilt angles or accelerations of the twodirections simultaneously as described above are synthesized in thevector manner and accordingly, a direction of maximum tilt angle or thedirection of acceleration on a two-dimensional surface and the magnitudevalues can be obtained.

Modified Forms of the First and Second Embodiments

The liquid sensors 1 and 1 a of the first and second embodiments may bemodified as follows. For example, when the plate-shaped main electrodes4 and 5 are fitted in the openings of the sides of the container, thegaps in the abutting portions of the main electrodes 4 and 5 and thenotches 19 and 20 are filled with the sealing agent 28, as shown in FIG.4. Further, the gaps between the outer peripheral faces of the mainelectrodes 4 and 5 and the sides of the notches 19 and 20 are alsofilled with the sealing agent 28. Thus, the main electrodes 4 and 5 arefixed to the cylindrical container 6 for fluid tightness.

In order that the fixing and the fluid tightness may be rendered morereliable, as shown in FIG. 10, recess-shaped, U-shaped and V-shapedgrooves 29 and 30 are formed, and spaces defined by the grooves 29 and30 and the main electrodes 4 and 5 are filled with the sealing agent 28for fluid tightness. As a result, since the main electrodes 4 and 5 arefixed to the cylindrical container 6 by the peripheral three sides, thefixation can be intensified and a liquid-leak preventing effect can beimproved.

Further, the metal electrode bar 8 serving as an auxiliary electrodeextends through the upper lid 11 and is fixed so that a distal endthereof is soaked sufficiently deep in the conductive liquid 7. Insteadof taking out the potential of the conductive liquid 7 in this way, anentire or a part of the lower lid 12 may be made of an electricallyconductive material thereby to serve as an auxiliary electrode and alead wire may be connected to the auxiliary electrode so that thepotential of the conductive liquid 7 is taken out.

Further, the plate-shaped main electrodes 4 and 5 fitted in the notches19 and 20 may have respective outer faces the entire of each of which iscovered with a sealing agent. Consequently, the outer faces of the mainelectrodes 4 and 5 can be protected.

Third Embodiment

A third embodiment of the present invention will be described withreference to the drawings. FIG. 11 is a longitudinal section of theliquid sensor of the embodiment, and FIG. 12 is a perspective viewshowing arrangement of electrodes in the container of the liquid sensor.

The liquid sensor 40 of the embodiment has a pair of main electrodes andcomprises a container 41, an electrically conductive liquid 42, a disc43, first and second lead terminals 44 and 45 and first and second mainelectrodes 46 and 47.

The container 41 is a substantially circularly cylindrical containerwith a closed end and is made of an electrically conductive material. Acorrosion-resistant metal such as stainless steel is used as theconductive material. An opening of the container 41 is closed by thedisc 43.

The closed container 48 comprising the container 41 and the disc 43 isfilled with an electrically conductive liquid 42 whose amount is equalto substantially one-half of an inside volume of the container. Theconductive liquid 42 is the same as described in the first embodiment.The upper inner space of the closed container 48 is filled with theinert gas.

The first and second lead terminals 44 and 55 extend through the disc 43and are fixed while being electrically insulated from the disc. The leadterminals 44 and 55 have distal ends which protrude into the closedcontainer 48 and to which the first and second main electrodes 46 and 47are mounted. The surfaces of portions of the lead terminals 44 and 55protruding into the container 48 are covered with an insulating resin soas not to come into electrical contact with the conductive liquid 42.Connecting portions of the lead terminals 44 and 55 and the mainelectrodes 46 and 47 are also covered with the insulating resin in thesame way thereby to be protected.

The main electrodes 46 and 47 are formed into the same shape of a shortstrip. The shape is selected so that a surface area of each electrode isincreased so that the capacitance is increased and accordingly an amountof change in the capacitance is rendered easier. The main electrodes 46and 47 are mounted so that short strip-shaped sides of them confronteach other and so that main electrode sides are parallel to each other,that is, mounted oppositely so that an imaginary line connecting the twolead terminals 44 and 55 is perpendicular to the sides of the respectivemain electrodes 46 and 47. When the closed container 48 is in a verticalstate, an upper one-third to one-half part of each of the mainelectrodes 46 and 47 is exposed over the surface of the conductiveliquid 42.

Each of the main electrodes 46 and 47 is made from an electricallyconductive material and has a surface formed with a thin silicon oxidefilm. A parallel plate capacitor is formed and has one of the mainelectrodes and the conductive electrode 2 serving as the other electrodewith the thin silicon oxide film as a dielectric being interposedbetween the electrodes. Since each main electrode has a surface formedwith the silicon oxide film, a silicon material such as single crystalsilicon, amorphous silicon, polycrystalline silicon or the like is usedas in the first embodiment. The silicon oxide film is also formed by anordinary method used in the IC production process, such as the thermaloxidation process, CVD process or the like as in the first embodiment.

The liquid sensor 40 of the embodiment can also be used as a tiltsensor, acceleration sensor and seismoscope as the liquid sensor 1 ofthe first embodiment. FIG. 13 shows the case where the liquid sensor 40is used as a tilt sensor. A contact area of the main electrode 46 withthe conductive liquid 42 is increased when the closed container 48 tiltsby a tilt angle θ from the vertical axis along an imaginary lineconnecting the lead terminals 44 and 55. Consequently, the capacitanceof the capacitor comprising the main electrode 46, silicon oxide filmand conductive liquid 42 is increased. Conversely, the capacitance ofthe capacitor comprising the main electrode 47, silicon oxide film andconductive liquid 42 is decreased. Accordingly, as described in thefirst embodiment, the value of tilt angle θ can be obtained when thedifference of capacitance is measured by the bridge circuit as shown inFIG. 7. In this case, the potential of the conductive liquid 42 is takenfrom the closed container 48 made from the conductive material.

FIG. 14 shows the case where the liquid sensor 40 is used as anacceleration sensor. In this case, too, the capacitance of the capacitorcomprising the main electrode 46, silicon oxide film and conductiveliquid 42 is increased, as in the case of FIG. 6 in the firstembodiment. Conversely, the capacitance of the capacitor comprising themain electrode 47, silicon oxide film and conductive liquid 42 isdecreased. Accordingly, the magnitude of acceleration applied to theclosed container 48 can be obtained when the difference of capacitanceis measured.

Fourth Embodiment

The liquid sensor 40 of the third embodiment can only measure a tiltangle or acceleration along the line perpendicular to the surfaces ofthe paired main electrodes. However, a liquid sensor of a fourthembodiment is expanded so as to be able to measure a tilt angle oracceleration along two directions intersecting each other in ahorizontal face.

FIG. 15 is a perspective view of the liquid sensor 40 a of the fourthembodiment, showing the arrangement of the main electrodes in thecontainer. The liquid sensor 40 a is provided with an additional pair ofmain electrodes 50 and 51 in the closed container 48. Four mainelectrodes 46, 47, 50 and 51 are mounted so that the surfaces of eachelectrode and an adjacent one meet each other at right angles, that is,so that an imaginary line connecting the lead terminals of each pairintersect each other and is perpendicular to the sides of the mainelectrodes mounted to the respective lead terminals.

According to the above-described construction, the liquid sensor canmeasure a tilt angle or acceleration along a line perpendicular to thesurfaces of the main electrodes 50 and 51 simultaneously with the tiltangle or acceleration along a line perpendicular to the surfaces of themain electrodes 46 and 47. More specifically, tilt angles oraccelerations along two directions intersecting each other in thehorizontal face can be measured simultaneously. Accordingly, the twotilt angles or two accelerations obtained simultaneously as describedabove are synthesized in the vector manner and accordingly, a directionof maximum tilt angle or the direction of acceleration on atwo-dimensional surface and the magnitude values can be obtained.

Modified Forms of the Third and Fourth Embodiments

The liquid sensors 40 and 40 a of the third and fourth embodiments maybe modified as follows. For example, in the case of the liquid sensor 40of the third embodiment, the main electrodes 46 and 47 may be mounted sothat the surfaces of them are co-planar as shown in FIG. 16. In thisstructure, too, when the closed container 48 tilts, a contact area ofeach main electrode with the conductive electrode 42 changes such that adifference of capacitance arises as shown in FIG. 17. Accordingly, atilt angle of the closed container 48 can be obtained by measuring thedifference. An acceleration can also be obtained in the same manner.

Further, in the fourth embodiment, the two pairs of main electrodes maybe mounted so that the surfaces of the four main electrodes 46, 47, 50and 51 adjacent to one another disposed radially while meeting eachother at angles of 90° as shown in FIG. 18. Consequently, as in thefourth embodiment, a tilt angle or acceleration can be measured alongtwo lines connecting the paired main electrodes. Further, this structurehas an effect that each main electrode limits an unfavorable rotationalmotion of the conductive liquid 42 in the closed container 48.

Further, in the third and fourth embodiments, the potential of theconductive liquid 42 is taken out from the closed container 48 made fromthe conductive material in the third and fourth embodiments. As shown inFIG. 19, however, an electrically conductive auxiliary electrode 52 maybe mounted so as to extend through the disc 43 closing the opening inorder to take out the potential.

Further, the liquid sensors 40 and 40 a are used with the discs 3 beinglocated at the bottom thereof in the third and fourth embodiments.However, the liquid sensors may be used upside down or while the discs 3are located at the top and the closed containers 48 are located at thebottom.

Other Modified Forms

In the hitherto described first to fourth embodiments, two opposed mainelectrodes are parallel to each other. However, the main electrodes neednot be parallel to each other so long as the difference of capacitancebetween the opposed electrodes can be detected by the bridge circuit.Additionally, the surface of each main electrode need not be flat. Forexample, each main electrode may be bar-shaped in the third and fourthembodiments. Further, in the first and second embodiments, in order thatthe capacitance may be increased by increasing the contact area with theconductive liquid, each main electrode protruding into the conductiveliquid may have a triangular or semicircular section.

Further, the electrodes a repaired in the hither to describedembodiments. However, a single main electrode may be provided. Forexample, a single main electrode is disposed at a position shifted fromthe center of the closed container. In this case, an immersion depth ofeach main electrode changes upon changes in the liquid surface of theconductive liquid, whereupon the capacitance between each main electrodeand the conductive liquid changes. A tilt or acceleration applied to theclosed container can be detected by measuring the aforesaid changes inthe capacitance. In this case, the value of capacitance can be measuredwhen a capacitor comprising the main electrodes, silicon oxide films andconductive liquid is connected to a side of a bridge circuit.

INDUSTRIAL APPLICABILITY

As described above, the liquid sensor in accordance with the presentinvention is suitable for use as a sensor detecting a tilt angle of anobject or a direction and magnitude of acceleration applied to an objectin the horizontal direction. The liquid sensor is further suitable foruse as a seismoscope.

1. A liquid sensor comprising: a container filled with an electricallyconductive liquid; an electrode including a part having a surface formedwith a dielectric film, the electrode being brought into contact withthe conductive liquid so that a change in a contact area of theelectrode with the conductive liquid with movement of a surface of theconductive liquid is measured from a change in a capacitance between theelectrode and the conductive liquid and so that a change in a tilt angleof the electrode to the liquid surface or an acceleration applied to thecontainer is detected from a measured value, wherein the dielectric filmcomprises a silicon oxide film.
 2. A capacitance type liquid sensorcomprising: a cylindrical closed container made from an electricallyinsulating material and having two sides parallel to each other, thesides having respective openings formed therein, the container beingfilled with a conductive liquid; a plurality of plate-shaped mainelectrodes each having at least one side formed with a silicon oxidefilm, the main electrodes being in abutment with said sides while thesilicon oxide films face an inside of the container, thereby closing theopenings; a sealing agent interposed in a gap between the mainelectrodes and said sides for fluid tightness respectively; and anauxiliary electrode provided in the container to be brought intoelectrical contact with the conductive liquid, wherein the conductiveliquid has an amount corresponding to substantially one half of acontent volume of the container.
 3. A capacitance type liquid sensorcomprising: a cylindrical closed container made from an electricalinsulator and having two sides parallel to each other and upper andlower openings closed by lids respectively, said sides having verticallyextending rectangular openings formed in the sides; a plurality ofnotches formed by cutting out corners located outside the container withrespect to all end faces of the openings so that plate-shaped membersare capable of being fitted in the notches; a plurality of plate-shapedelectrodes having at least one side formed with silicon oxide films andfitted in the notches so as to abut against bottoms of the notches withthe oxide films facing an inside of the container; a sealing agentprovided for fluid tightness in gaps defined between the bottoms of thenotches and the plate-shaped electrodes and gaps defined between outerperipheral sides of the plate-shaped electrodes and sides of thenotches; an electrically conductive liquid having an amount equal tosubstantially one-half of an inside volume of the container and fillingthe container; and an auxiliary electrode bar made from a metal andextending through substantially a central part of the upper lid so thata distal end thereof is immersed sufficiently deep in the conductiveliquid.
 4. The capacitance type liquid sensor according to claim 2,wherein the cylindrical container is formed into a square cylindricalshape and has two pairs of parallel sides, the four parallel sideshaving openings closed by plate-shaped main electrodes having siliconoxide films respectively.
 5. The capacitance type liquid sensoraccording to claim 3, wherein the cylindrical container is formed into asquare cylindrical shape and has two pairs of parallel sides, the fourparallel sides having openings closed by plate-shaped main electrodeshaving silicon oxide films respectively.
 6. The capacitance type liquidsensor according to claim 3, wherein the bottoms of the notches areformed with grooves respectively and the sealing agent fills for fluidtightness spaces defined between the grooves and the plate-shaped mainelectrodes respectively.
 7. The capacitance type liquid sensor accordingto claim 3, wherein an entire or a part of the lower lid is made from anelectrically conductive material to serve as an auxiliary electrode,instead of the auxiliary electrode.
 8. The capacitance type liquidsensor according to claim 2, wherein an entire side of the plate-shapedmain electrode facing an outside of the container is covered with asealing agent.
 9. The capacitance type liquid sensor according to claim2, wherein the sealing agent comprises a glass having a low meltingpoint.
 10. A capacitance type liquid sensor comprising: a closedcontainer; an electrically conductive liquid having an amount equal tosubstantially one-half of an inside volume of the container and fillingthe container; a pair of lead terminals extending through one of ends ofthe container and fixed to the container so as to be electricallyinsulated from the container; a plurality of main electrodes havingsurfaces formed with silicon oxide films and mounted on distal ends ofthe lead terminals, respectively, the main electrodes being provided sothat parts of the main electrodes are located on a liquid surface of theconductive liquid when the container is stationary; and an auxiliaryelectrode electrically conductively brought into contact with theconductive liquid.
 11. The capacitance type liquid sensor according toclaim 10, wherein two pairs of the lead terminals extend through saidone end of the container and are fixed to the container, instead of onepair, the main electrodes are mounted on distal ends of the leadterminals respectively, and imaginary lines connecting centers of therespective paired main electrodes intersect perpendicularly to eachother.
 12. The capacitance type liquid sensor according to claim 10,wherein the main electrodes are formed into a shape of a short strip andtwo main electrode sides of each pair are opposed to each other so thatimaginary lines connecting centers of the respective paired mainelectrodes are perpendicular to the sides of the respective paired mainelectrodes.
 13. The capacitance type liquid sensor according to claim10, wherein the main electrodes are formed into a shape of a short stripand are disposed radially so that angles made by the surfaces of themain electrodes adjacent to each other are equal to each other.
 14. Thecapacitance type liquid sensor according to claim 10, wherein theauxiliary electrode is the closed container made from an electricallyconductive material.
 15. The capacitance type liquid sensor according toclaim 10, wherein the auxiliary electrode extends through one of ends ofthe container and is fixed to the container.
 16. The capacitance typeliquid sensor according to claim 2, wherein each main electrode is madefrom any one of single crystal silicon, amorphous silicon, andpolycrystalline silicon.
 17. The capacitance type liquid sensoraccording to claim 2, wherein the conductive liquid is made from any oneor a combination of two or more of an alcohol group including methylalcohol, ethyl alcohol and isopropyl alcohol, a ketone group includingacetone and methyl ethyl ketone and an ether group including diethyleneglycol mono-butyl ether with addition of an electrolyte includinglithium nitrate or potassium iodide.
 18. The capacitance type liquidsensor according to claim 2, wherein the closed container contains aninert gas.
 19. The capacitance type liquid sensor according to claim 5,wherein the bottoms of the notches are formed with grooves respectivelyand the sealing agent fills for fluid tightness spaces defined betweenthe grooves and the plate-shaped main electrodes respectively.
 20. Thecapacitance type liquid sensor according to claim 5, wherein an entireor a part of the lower lid is made from an electrically conductivematerial to serve as an auxiliary electrode, instead of the metalauxiliary electrode.
 21. The capacitance type liquid sensor according toclaim 3, wherein an entire side of the plate-shaped main electrodefacing an outside of the container is covered with a sealing agent. 22.The capacitance type liquid sensor according to claim 4, wherein anentire side of the plate-shaped main electrode facing an outside of thecontainer is covered with a sealing agent.
 23. The capacitance typeliquid sensor according to claim 5, wherein an entire side of theplate-shaped main electrode facing an outside of the container iscovered with a sealing agent.
 24. The capacitance type liquid sensoraccording to claim 3, wherein the sealing agent comprises a glass havinga low melting point.
 25. The capacitance type liquid sensor according toclaim 4, wherein the sealing agent comprises a glass having a lowmelting point.
 26. The capacitance type liquid sensor according to claim5, wherein the sealing agent comprises a glass having a low meltingpoint.
 27. The capacitance type liquid sensor according to claim 11,wherein the main electrodes are formed into a shape of a short strip andtwo main electrode sides of each pair are opposed to each other so thatimaginary lines connecting centers of the respective paired mainelectrodes are perpendicular to the sides of the respective paired mainelectrodes.
 28. The capacitance type liquid sensor according to claim11, wherein the main electrodes are formed into a shape of a short stripand are disposed radially so that angles made by the main electrodesides adjacent to each other are equal to each other.
 29. Thecapacitance type liquid sensor according to claim 11, wherein theauxiliary electrode is the closed container made from an electricallyconductive material.
 30. The capacitance type liquid sensor according toclaim 11, wherein the auxiliary electrode extends through one of ends ofthe container and is fixed to the container.
 31. The capacitance typeliquid sensor according to claim 3, wherein each main electrode is madefrom any one of single crystal silicon, amorphous silicon, andpolycrystalline silicon.
 32. The capacitance type liquid sensoraccording to claim 4, wherein each main electrode is made from any oneof single crystal silicon, amorphous silicon, and polycrystallinesilicon.
 33. The capacitance type liquid sensor according to claim 5,wherein each main electrode is made from any one of single crystalsilicon, amorphous silicon, and polycrystalline silicon.
 34. Thecapacitance type liquid sensor according to claim 10, wherein each mainelectrode is made from any one of single crystal silicon, amorphoussilicon, and polycrystalline silicon.
 35. The capacitance type liquidsensor according to claim 11, wherein each main electrode is made fromany one of single crystal silicon, amorphous silicon, andpolycrystalline silicon.
 36. The capacitance type liquid sensoraccording to claim 3, wherein the conductive liquid is made from any oneor a combination of two or more of an alcohol group including methylalcohol, ethyl alcohol and isopropyl alcohol, a ketone group includingacetone and methyl ethyl ketone, an ether group including diethyleneglycol mono-butyl ether and the like with addition of an electrolyteincluding lithium nitrate, potassium iodide or the like.
 37. Thecapacitance type liquid sensor according to claim 4, wherein theconductive liquid is made from any one or a combination of two or moreof an alcohol group including methyl alcohol, ethyl alcohol andisopropyl alcohol, a ketone group including acetone and methyl ethylketone, an ether group including diethylene glycol mono-butyl ether andthe like with addition of an electrolyte including lithium nitrate,potassium iodide or the like.
 38. The capacitance type liquid sensoraccording to claim 5, wherein the conductive liquid is made from any oneor a combination of two or more of an alcohol group including methylalcohol, ethyl alcohol and isopropyl alcohol, a ketone group includingacetone and methyl ethyl ketone, an ether group including diethyleneglycol mono-butyl ether and the like with addition of an electrolyteincluding lithium nitrate, potassium iodide or the like.
 39. Thecapacitance type liquid sensor according to claim 10, wherein theconductive liquid is made from any one or a combination of two or moreof an alcohol group including methyl alcohol, ethyl alcohol andisopropyl alcohol, a ketone group including acetone and methyl ethylketone, an ether group including diethylene glycol mono-butyl ether andthe like with addition of an electrolyte including lithium nitrate,potassium iodide or the like.
 40. The capacitance type liquid sensoraccording to claim 11, wherein the conductive liquid is made from anyone or a combination of two or more of an alcohol group including methylalcohol, ethyl alcohol and isopropyl alcohol, a ketone group includingacetone and methyl ethyl ketone, an ether group including diethyleneglycol mono-butyl ether and the like with addition of an electrolyteincluding lithium nitrate, potassium iodide or the like.
 41. Thecapacitance type liquid sensor according to claim 3, wherein the closedcontainer contains an inert gas.
 42. The capacitance type liquid sensoraccording to claim 4, wherein the closed container contains an inertgas.
 43. The capacitance type liquid sensor according to claim 5,wherein the closed container contains an inert gas.
 44. The capacitancetype liquid sensor according to claim 10, wherein the closed containercontains an inert gas.
 45. The capacitance type liquid sensor accordingto claim 11, wherein the closed container contains an inert gas.